The demand for bandwidth is scaling up to unprecedented levels. With the 100 Gb Ethernet (100 GbE) standard on the horizon, serial data communication rates will eventually outpace the single-channel capabilities of telecom transport interfaces. In five years, the transport of terabits of data generated from multitudes of data gathering and processing nodes, will need to be managed and delivered, on demand, to users at secure campuses. The cost-effective use of existing public dark fiber, i.e. unused installed fiber, and the emerging transparent reconfigurable optical add-drop multiplexer (ROADM)-based networks create a compelling case for photonic layer security (PLS) for high bandwidth needs where advanced encryption systems (AES) is not practical.
The success and widespread use of Code Division Multiple Access (CDMA) in the wireless domain has renewed interest in exploring the use of CDMA in the optical domain as well. However, optical CDMA (OCDMA) presents a very different set of challenges. Initially, these challenges included spectral efficiency, which is typically low in OCDMA, inter-symbol interference, and susceptibility to network impairments. Significant progress has been made since the start of a Defense Advanced Research Projects Agency (DARPA) sponsored program addressing the shortcomings of OCDMA. To the extent that spectral efficiency comparable to Wavelength-Division Multiplexing (WDM) networks is achieved, OCDMA networking has attractive features based on the underlying property that connectivity is managed through the use of codes like telephone numbers as compared with wavelength exchange in WDM networks.
Three distinct approaches have been the subject of much recent research, a potentially asynchronous time chips frequency bins or fast frequency hopping (FFH), a potentially asynchronous coherent time chips phase bins, and a synchronous coherent phase chips frequency bins or spectral phase encoding (SPE).
The attraction of SPE-OCDMA-based network lies in the prospect for providing security for multi-Gb/s data streams at the photonics layer. At the same time, concerns remain regarding quantifying the level of robustness of SPE-CDMA against eavesdropping. PLS offered by SPE-OCDMA has been historically viewed as security by obscurity, and thus the security robustness of SPE-OCDMA has been controversial since the theoretical paper by V. J. T. Shake entitled “Confidentiality Performance of Spectral-Phase-Encoded Optical CDMA,” (published in IEEE Journal of Lightwave Technology, April 2005, pp. 1652-1663) and experimental analysis by Weiner et al. (i.e., “Experimental investigation of security issues in OCDMA: a code-switching scheme,” Elec. Lett., Vol. 41, No. 14, Jul. 7, 2005; and “Experimental investigation of security issues in OCDMA,” JLT, vol. 24, pp. 4228-4234, 2006), which considered limitations of SPE-OCDMA security in a particular case.
The case considered in the Shake and Weiner et al. references is limited to a situation where a single channel is present, and as stated by Shake “allowing an eavesdropper to isolate individual user signals”.